Among various phenolic compounds, 2,4′-dihydroxydiphenylsulfone (hereinafter occasionally referred to as 2,4′-DDS) has recently attracted attention because of its great utility as a color developer for heat-sensitive paper.
Known methods for producing 2,4′-DDS include the separation and purification of 2,4′-DDS from mixtures containing 2,4′-DDS and 4,4′-dihydroxydiphenylsulfone (hereinafter occasionally referred to as 4,4′-DDS).
Moreover, it is known that when 4,4′-DDS is heated in a phenol solvent in the presence of an acid catalyst, isomerization occurs, thereby producing a mixture of dihydroxydiphenylsulfone isomers containing 4,4′-DDS and 2,4′-DDS in a weight ratio of about 75 to about 25 (isomerization equilibrium) (Journal of the Chemical Society of Japan, 1985 (1), 70–74). However, since this isomerization reaction requires 20 hours or longer to reach isomerization equilibrium even under heating conditions of 190° C. or higher, it is difficult to employ this isomerization reaction industrially.